A buffet restaurant is a buffet restaurant. Help yourself is help yourself or self-service. It's easy to understand if you give a complete example: "This restaurant offers lunch buffet, please go ahead and help yourself. This restaurant offers lunch buffet, please go ahead help yourself. "A la carte" is more difficult to understand, it is A la carte. or by entrees. "A la carte" is often seen in American menu, but it is not easy to learn in China. It's not originally English, but was borrowed from French earlier. You can look up a bunch of ins and outs at www.wikipedia.org.